有关多表Left join的优化
SELECT  
COUNT(DISTINCT T1.A1) + COUNT(DISTINCT T2.B1) + 
COUNT(DISTINCT T3.C1) FROM T1  
LEFT JOIN T2 on T1.A1 = T2.A1  
LEFT JOIN T3 on T1.A1 = T3.A1  
LEFT JOIN T4 on T3.C1 = T4.C1  

GROUP BY T1.A2, T1.A3 


上面这个SQL效率太差了，
T1,T2,T3,T4表达到5千条的时候，需要两，三分钟
希望大家帮忙优化一下 
多谢大家帮帮忙吧！

------解决方案--------------------
LEFT JOIN T4 on T3.C1 = T4.C1

不需要，先去掉
------解决方案--------------------
需要这样的连接,除了索引上优化,没什么可优化的了.
------解决方案--------------------
在A1,C1上建索引就可以了，5千条不多
------解决方案--------------------

SQL code

--try:
SELECT  
COUNT(DISTINCT T1.A1) + COUNT(DISTINCT T2.B1) + 
COUNT(DISTINCT T3.C1)  FROM T1  
inner JOIN T2 on T1.A1 = T2.A1  
inner JOIN T3 on  T1.A1 = T3.A1  
GROUP BY T1.A2, T1.A3